Steroid hormones have been shown to accelerate the appearance of pulmonary surface-active phospholipid. I propose a morphologic and morphometric study of the immediate and long-term effects of intrauterine, intrafetal injection of prednisolone or of estradiol on the lung, kidney and sex organs of newborn rabbits. The effect of these hormones on the major pathways of surface-active phospholipid synthesis will be measured by the incorporation of labelled precursor into alveolar wash and lung tissue lipids. The biochemical characterization of lung alveolar wash and tissue lipid classes will be carried out separately. The presence and localization of steroid receptors will be studied by autoradiography.